Amplifier system



July 10, 1956 INVENTORS ATTORNEY United States Patent to the United States of America as represented by the Secretary of the Air Force Application December-'24, 1952, Serial No. 327,833

8 Claims; (Cl.

This invention relates to an electronic amplification system, and more particularly to an improved amplification system of the type suitable for charging or discharging capacitive loads.

In applying high frequency pulse type signals to a capacitive load such as the input to a number of tubes or a long transmission line, it is desirable to drive the load without distortion of the pulse waveshapes. In order to achieve this, the load may be charged through an electron discharge path provided by a vacuum tube and may be discharged through an electron discharge path provided by a second vacuum tube. The charge and discharge tubes may be connected in series, with the capacitive load connected across one of them. In one such arrangement that has been developed, the charge and discharge tubes are controlled from separate signal" sources to charge and discharge the load. In another arrangement for charging and discharging, asingle signal source controls one of the tubes which in turn drives the other tube. Where a large positive-going pulse is to beapplied to a capacitive load, the tubes may be operated so as to be conducting in the standby or quiescent condition, and driven to cut oil" by a negative-going input pulse. In that case, one of the tubeshas a large quiescent voltage drop across it, and at the same time, draws a large quiescent anode current. As a result, there is a large, quiescent, anode power dissipation, and a large expensive power tube is needed.

Therefore, it is an object of this invention to provide an improved amplifier system.

Another object of this invention is to provide a novel and inexpensive amplifier system for driving a capacitive load which leaves the driving waveshape substantially unaffected.

Still another object of this invention is to provide a novel amplifier system which is economical in the components required, and has an improved power efiiciency.

These and other objects of this invention are achieved in one embodiment of this invention in" which a first and second electron discharge tube of the' pentode type are connected in s'erics. The cathode of the first tube is connected to the anode of the second tube through a resistance and an inductance in series. The control grid of the first tube is connected to the anode of the second tube, and the anode of the first tube is connected to' the positive side of a source of operating potential. The screen grids of both tubes are connected to one end of a resistance element, the other end of which is conne'c't ed to the source of operating potential. The control grid of the second tube is normally at zero' bias and it receives negative-going input pulses. The output is taken across the second tube.

in the standby condition, both tubes are conducting. The ca'p'aci'tive l'o'adis charged to a small voltage, which is' the quiescent voltage drop across the second tube, so that it' essentially discharged. A negative pulse applied to the control grid o'f-th' second tube cuts that tube on. The first tube now at zero bias, conducts 2,754,376 Patented- July 10,-- 1956 heavily to charge the capacitive load to the value of the operating potential, so that a positive-going wave is developed across the load. At the termination ofthe input pulse, conduction in the second tube is abruptly reestablished, and the capacitive load discharges through the second tube. The voltage drop due to current now through the resistor places a negative bias on the control grid of the first tube so that it is cut off during discharge of the load.

In the standby condition, the second tube isconducting, but the voltage across it is small so thatthe power dissipation is small; The current drawn by the screen of the second tube produces a voltage dropacross the screen resistor, and thus serves to reduce the voltage at the screen of the first tube; Thus, there isa corre sponding decrease in quiescent anode current in the first tube. Therefore, the quiescent power dissipation in the first tube is made to be small even though the voltage across it is large. However, when the second tube is cut ed, the voltage at the screen of the first tube rises; and that tube can conduct heavily. In this way, a" low power rated tube may be used to function as a power tube. This invention may also be applied to other cir-' cuits than that described above.

The novel features of the invention, as well as the invention itself, both as to its organization and method of operation is best understoodfrom' the following description when read together with the accompanying drawing in which:

Figure 1 is a schematic diagram of a circuit embodying this invention; and v Figure 2 is a schematic diagram of a modified circuitembodyingthis invention".

Referring now to Figure 1, there is shown a circuit embodying the features of this invention. A first and second electron discharge tube 10, I2 of the pentode type are connected in series, with the cathode 14 of the first tube 10 connected through a resistor 16 and inductor 18 to the anode 26 of the secondtube 12. The control grid 22 of the first tube 10 is connecteddirectly to the anode 20 of the second tube 12. The control grid-24 of the second tube 12 is" connectedto the cathode 26' of that tube through a grid resistor 28 and it isalso connected' to an input terminal 30. The positive side of asource of operating potential is applied to the anode 32 of the first tube 10. The screen grids 34, 36 of both tubes 10, 12- are connected to" one end of acommon screen resistor 38, the other end of which is connected to B+. The output ofthe circuitistakenfrom a ter minal 4% connected to the cathode 14 of the first tube 10. One side of a capacitive load 42 (shown ina broken line rectangle) is connected to the output terminal 40, and the other side of the load isconne'ctedto'th'e cathode 26 of the second tube 12.

In the standby condition of the amplifier, the second tube 12 is at zero bias, and is therefore, conducting. Therefore, the capacitive load 42 is normally discharged. The first tube 10 is also conducting, although the control grid 22 is biased to some negative value due to the volt-- age drop acros the cathode resistor 16. When ane'ga-' tive-going input pulse is applied to the control grid 24 of the second tube 12, current-flow through thattube is cut off. The first tube It) continues to conduct but now at zero bias, and the capacitive load 42 is charged quickly through that tube 10 to substantially the potential' of B+. When the input-pulse terminates, the'sec ond tube 12 starts to conductabruptly, andthe capacitiveload 42 discharges through that tube 12 and through the resistor 16 and inductor 18 in the discharge'path. The

voltage drop across the resistor 16 in the; discharge path due to this current fiow placesa negative bias on the control grid 22 of the first tube 10 which is suflicient to cut that tube off until the capacitive load 42 is fully discharged. In this way, a negative-going input pulse is inverted and amplified to produce a positivegoing output pulse across the capacitive load 42 with sharp trailing and leading edges.

. The inductor 18 in the discharge path serves to sharpen the leading and trailing edges of the output wave. VJhen the second tube 12 is abruptly cut ofii, a negative voltage momentarily appears across the inductor 18 due to the decreasing current therethrough. This make the bias at the control grid 22 positive, and produces an increase in charging current. When conduction is reestablished in the second tube 12, a large negative voltage is placed on the control grid 22 more quickly to cut ofi thefirst tube. Thus, discharge of the load is facilitated.

In the standby condition of the amplifier, there is a relatively small voltage across the second tube 12 so that there is a small quiescent anode power dissipation in that tube. The first tube 10, however, has a relatively large voltage 'drop across it, at least equal to the amplitude of the output pulse. The quiescent power dissipation in the anode 32 of this tube is reduced by controlling the quiescent anode current. This is done by connecting the 'screen grids 34, 36 of both tubes to the common screen anode current of the first tube 10. Therefore, quiescent anode power dissipation in the first tube 10 is made to be small, even though the voltage across it is relatively large. During discharge of the capacitive load 42, the second tube 12 is conducting and drawing screen current so that the voltage at the screen grid 34 of the first tube 10 is reduced. This has the effect of reducing the grid base of the first tube lfi, that is the grid voltage change required for tube cutoff is reduced. Therefore, the resistor 16in the discharge path may be made smaller, which results in an increase in the discharge currentand a decrease in the discharge time. screen control action for the duration of the input pulse, because screen current in the second tube 12 is cut ofi at that time. Therefore, the screen voltage of the first tube 10 rises when the second tube is cut ofi, the first tube conducts more heavily, and the capacitive load is quickly charged, as desired. Since the quiescent power dissipation in the first tube 10 is small, a low power rated tube may be used which can adequately supply the power demands for charging the capacitive load.

The capacitive load may also be connected across the first tube 10 with no change in the circuit. The operation is the same, except that the load charges through the second tube and discharges through the first tube.

In Figure 2, this invention is shown and embodied in a modified form of the circuit described above. In this circuit, a first pentode 50 has its cathode 52 directly connected to the anode 54 of a second tube 56, and also connected to the anode 58 of a third tube 60 through a resistor 62 and inductor 64 in series. The control grid 66 of the first tube 50 is directly connected to the anode 58 of the third tube 6%). The anode 68 of the first tube 50 is connected to B+. The screen grids 70, 72 of the first and second tubes 50, 56 are connected to one end of a common screen resistor 74, the other end of which is connected to B+. The control grids 76, 78 of the second and third tubes 56, were connected to their respective cathodes 80, 82 through separate grid resistors 84, 86. The control grids 76, 78 of these tubes are also coupled to a common input terminal 88 though separate capacitors. The cathode 82 of the third tube 60 is connected to ground, and the cathode t of the second tube 56 is connected to an intermediate potential level.- The screen grid 90 of the third tube 6%) is con- Furthermore, there is no nected to an intermediate potential level through a series resistor 92. One side of a capacitive load 94 is con-' nected to an output terminal96 at the cathode of the first tube 50, and the other side is connected to ground, so that the load 94 is connected across the third tube 60.

At standby condition, the third tube 60 is effectively at zero bias so that it is conducting. The first tube is also conducting, although its grid has a slightly negative bias. The second tube 56 has its anode at a lower potential than its cathode so that it is not conducting, although normally at zero effective bias. However, screen current is drawn through the second tube 56. When a negative-going input pulse is applied to the input terminal 88, the third tube is cut ofi, and both screen and anode current in the second tube 56 are cut off. The first tube 5% which now has a highly positive screen grid and a zero bias control grid, conducts heavily to charge the capacitive load 94. At the termination of the pulse, the anode of the second tube 56 is substantially at B+, and it starts to conduct to discharge the capacitive load.

There is also current flow through the third tube 60, and

through the resistor 62 and inductor 64 in the discharge path, of sufficient magnitude to place a negative voltage on the control grid of the first tube 50 and cut that tube on. The second tube 56 continues to conduct until the potential at its anode is approm'mately equal to that of its cathode, when anode current is cut off. The third tube 60, however, continues to conduct to complete the discharge of the capacitive load 94.

Here again, the connection of the screen grids 70, 72 of the first and second tubes 5%, S6 to a common screen resistor reduces the quiescent power dissipation in the anode 68 of the first tube 56. In standby condition, there is screen current drawn by the second tube 56. As described above, this reduces the voltage at the screen grid of the first tube, which in turn reduces the quiescent anode current of that tube. Since the screen current in the second tube 56 is cut oifduring the period of the input pulse, the screen voltage in the first tube 50 rises. Thus, the first tube conducts more heavily to charge the load. As in the circuit embodying this invention shown in Figure 1, small size tubes may be used electron discharge tube each having anode, cathode, control grid and screen grid electrodes, means connecting said first tube cathode to said second tube anode,means for applying an operating'potential across both of said tubes, means for biasing both of said tubes to be conducting in standby condition, and means for reducing anode current flow in said first tube when said second tube is conducting including an impedance element, means connecting the screen grids of both of said tubes to one end of said impedance element, and means for applying an operating potential to the other end of said impedance element, the magnitude of said impedance element being such that the screen voltage of said first tube is substantially reduced when screen current is drawn through said impedance element by said second tube, whereby standby power dissipation is minimized.

2. An amplifier circuit comprising a first and second electron discharge tube each having anode, cathode, control' grid and screen gridelectrodes, means coupling said first tube control grid and said first tube cathode to said second tube anode, both of said tubes being biased to be conducting in standby condition, and means for reducing anode current flow in said first tube responsive to said second tube conducting including an impedance element, means connecting the screen grids of both of said tubes to one end of said impedance element, and means for applying an operating potential to the other end of said impedance element, the magnitude of said impedance element being such that the screen voltage of said first tube is substantially reduced when screen current is drawn through said impedance element by said second tube, whereby standby power dissipation is minimized.

3. In an amplifier circuit wherein a first electron discharge tube charges a capacitive load and a second electron discharge tube discharges the capacitive load responsive to input signals, wherein each of said tubes has anode, cathode, control grid and screen grid electrodes, and wherein said first tube cathode is coupled to said second tube anode, the improvement in said circuit of means for reducing anode current flow in said first tube when said second tube is conducting including an impedance element, means coupling both of said screen grids to one end of said impedance element, and means for applying an operating potential to the other end of said impedance element, said impedance element being connected so that all the current therethrough is applied to said screen grid electrodes, whereby the screen voltage of said first tube is substantially reduced when said second tube draws screen current through said impedance element and power dissipation is minimized,

4. In an amplifier circuit wherein a first and second electron discharge tube, each has anode, cathode, control grid and screen grid electrodes, wherein means biases both of said tubes to conduction in standby condition, and wherein said tubes are connected in series in a power supply circuit and said first tube is connected to be conducting when said second tube is cut ofi and said second tube is connected to be conducting when said first tube is cut off, the improvement in said circuit comprising means for reducing anode current flow in said first tube only when said second tube is conducting, including a resistance element, means connecting the screen grids of each of said tubes to one end of said resistance element, and means for applying an operating potential to the other end of said resistance element, whereby standby power dissipation is minimized.

5. An amplifier circuit comprising a first and a second electron discharge tube each having anode, cathode, con trol grid and screen grid electrodes, means connecting said first tube cathode and control grid to said second tube anode, a resistance element, means connecting the screen grids of both of said tubes to one end of said resistance element, means for applying an operating potential to the other end of said resistance element, means for applying a tube-conductive bias potential to the control grid of said second tube in standby condition, said resistance element being connected so that all the current therethrough is applied to said screen grids, whereby the screen voltage of said first tube is substantially reduced when said second tube draws standby screen current through said impedance element and standby power dissipation is minimized, an input terminal coupled to the control grid of said second tube and an output terminal coupled to the cathode of said first tube.

6. An amplifier circuit as recited in claim 5 wherein said means connecting said first tube cathode to said second tube anode includes an inductance element.

7. An amplifier circuit comprising a first, second and third electron discharge tube each having anode, cathode and control grid electrodes, means coupling said first tube cathode to the anodes of said second and third tubes, means coupling said first tube control grid to said third tube anode, each of said first and second tubes having a screen grid electrode, a resistance element, means coupling both of said screen grids to one end of said resistance element, means for applying an operating potential to the other end of said resistance element, means for applying a tube-conductive bias potential to the control grids of said second and third tubes in standby condition, an input terminal coupled to the control grids of said second and third tubes, and an output terminal connected to said first tube cathode.

8, An amplifier comprising first and second electron discharge tubes, each having anode, cathode, control grid and screen grid electrodes, means connecting the cathode of one of said tubes to the anode of the other of said tubes, and means for reducing anode current flow in said first tube when said second tube is conducting including an impedance element, and means of applying an operating potential to one terminal'of said impedance element, said screen grids of both tubes being connected together and to another terminal of said impedance element.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,200 Dow Apr. 11, 1939 2,211,914 Soller Aug. 20, 1940 2,232,212 Cary Feb. 18, 1941 2,323,211 Faltico June 29, 1943 2,423,931 Etter July 15, 1947 2,424,893 Mansford July 29, 1947 FOREIGN PATENTS 349,483 Great Britain May 26, 1931 529,864 Great Britain Nov. 29, 1940 

